Laser Ultrasonic Sensor Streamlines Papermaking Process

Hoping to save the paper-manufacturing industry millions of dollars in energy costs, Environmental Energy Technologies Division scientists have developed a laser ultrasonic sensor that measures paper's flexibility as it courses through a production web at up to 65 miles per hour.

"We're measuring the elastic properties of paper at manufacturing speeds using a non-contact, non-destructive monitor," said EETD's Paul Ridgway.

Mead Paper Co. Gives OIT-Developed LUS Technology a Positive Rating

Under OIT's Forest Products Industries of the Future program, the Lawrence Berkeley National Laboratory and the Institute of Paper Science and Technology jointly developed an innovative Laser Ultrasonic Sensor (LUS) to measure paper bending stiffness and shear rigidity during the papermaking process. The sensor was recently evaluated successfully on a pilot paper coating machine at Mead Paper Co. in Chillicothe, Ohio. Six different paper grades were used, ranging from relatively lightweight copy paper to heavy linerboard. Excellent LUS signals were obtained even at machine speeds up to 5000 ft/min, or about a mile per minute. No laser marks were visible on the paper. The LUS technology meets a major need of U.S. paper mills because a critical measurement normally performed off-line after production can be performed in real-time during the manufacturing process. Such a sensor can save the paper industry millions of dollars in energy and other costs by reducing the production of below-specification paper. For further information, contact OIT's Clearinghouse 1-800-862-2086.

Last summer, Ridgway and colleagues tested the laser ultrasonic sensor at a Mead Paper Company mill in Ohio. They installed the sensor on a pilot paper-coating machine and ran six paper grades—ranging from copy paper to heavy linerboard—through the web press. The sensor's signals remained excellent, even at paper speeds up to 5000 feet per minute, and the laser didn't damage the paper. The effects of the papers' moisture, tension, basis weight, and speed on the measurements were also examined.

"The Mead test demonstrated the instrument works in an industrial setting," Ridgway said. "It's a successful step toward a mill trial on a paper-making machine in which the environment will be much harsher. It will be hotter and wetter, and there will be more vibrations and fiber debris in the air."

The sensor is part of the Department of Energy's (DOE) Agenda 2020, a collaboration between the wood, paper, and forestry industry, launched in 1994 by the DOE to improve the industry's energy and resource efficiency. To understand how the sensor contributes to this initiative, consider how paper is currently evaluated. After it's manufactured, a small sample of a three-ton paper roll is manually analyzed for its mechanical properties by observing how it bends. If the sample doesn't meet specifications, the entire roll is scrapped or sold as an inferior grade. To avoid this costly mistake, manufacturers often over-engineer paper, erring on the side of caution and using more pulp than necessary, to ensure the final product isn't substandard. Not only does this consume more raw materials, it consumes more energy: The more pulp used per unit of paper, the more heat is required during the drying phase, which even in the most efficient mills requires an enormous amount of energy.

Rather than rely on post-production evaluation and hope for the best, Ridgway and colleagues have developed a sensor that measures flexibility on the fly, in real-time. It also conducts the measurements without touching the paper, an important advantage, given that at 30 meters per second, the slightest contact can mar light-weight grades such as copy paper and newsprint. This represents an improvement over contact transducers, another real-time evaluation tool that measures paper's tensile elasticity by placing an ultrasound head directly onto the paper as it's coursing through the web. Because it touches the paper, this technique can only be used with thicker stock.

A full-scale pilot test of the laser ultrasonic sensor is scheduled for the summer of 2003, Ridgway said. And further in the future, the sensor could provide quality-control safeguards and real-time process information for feedback process control in any manufacturing process involving thin, moving sheets, such as sheet metals, sheet plastics, polymeric materials, and glass. In addition, the sensor's auspicious Mead Paper Company field test represents a Berkeley Lab success under the auspices of the Laboratory Coordinating Council (LCC). The LCC was established in 1995 by the DOE Office of Industrial Technologies to merge the research and development capabilities of the 16 national labs and research facilities with the process needs of nine major industries: agriculture, aluminum, chemical, forest products, glass, metalcasting, mining, petroleum, and steel. See sidebar.

It works by essentially bringing the national labs under one roof. Rather than approach each lab individually, industry representatives can approach the LCC with a design need. The LCC, in turn, matches the industry project with the most appropriate lab. This gives American industry direct access to the entire DOE lab community at once. And by more efficiently pairing the national labs' vast research resources with the private sector, the LCC enables industry to become more resource- and energy-efficient, as well as more competitive in the global marketplace. As such, the DOE's Agenda 2020, which coupled the Environmental Energy Technologies Division with the paper industry's need for a non-contact paper sensor, is one of several industry-specific agendas designed to mesh industry needs with national lab know-how.

In rough terms, the sensor measures the time it takes ultrasonic shock waves to propagate from a the laser-induced excitation point to a detection point only millimeters away. The velocity at which the ultrasound waves travel from the ablation point through the paper to the detection point is theoretically related to two elastic properties: bending stiffness and out-of-plane shear rigidity. More specifically, a detection beam from a commercially available Mach-Zender interferometer is directed toward a quickly rotating mirror. As the mirror spins, the beam is reflected in a circular pattern much like a lighthouse's beam. During a portion of each revolution, the beam meets the paper as it courses along the production belt and remains with the paper until the beam's arc leaves the paper's plane. Think of the lighthouse beam momentarily tracking a speedboat as it races parallel to shore. Because both the beam and the paper are moving at the same speed, the detection beam remains at the same point on the paper.

An optical encoder determines when the detection beam is perpendicular to the paper, at which time a specially designed adjustable delay circuit fires the pulsed neodymium-yttrium-aluminum-garnet laser. This microsecond pulse causes a microscopic thermal expansion or ablation on the paper, which is too small to mar the paper and affect how it absorbs ink, but strong enough to send ultrasonic shock waves through the sheet. The waves propagate through the paper until they're registered by the detection beam. Because the laser is synchronized to fire only when the detection beam is perpendicular to the paper, the distance between the ablation point and detection point is known, and the wave's speed can be calculated.

— Don Krotz

For more information, contact:

Rick Russo

(510) 486-4258; fax (510) 486-7303

RERusso at lbl dot gov

This work is supported by the U.S. Department of Energy's Office of Industrial Technologies and Mead Paper Company.

Dan Krotz is a writer in Berkeley Lab's Public Information Department.